There are a large number of products and applications in which it is desirable and advantageous to reinforce a basic substrate material with a secondary material such as a string, thread, strand or tape that exhibits a relatively high tensile strength as compared to the basic substrate material. When the secondary material is securely bonded to the substrate, the composite structure exhibits the fundamental properties of the base substrate material with the increased tensile strength properties of the secondary material. Examples of such composite structures include tear tapes for use with multiwall bags and corrugated cartons and reinforced paper products such as tapes and corrugated paper. The basic substrate/strand combining process also finds significant commercial usage in industrial product fabrication processes such as in the joining of veneers and in laminating various kinds of substrates, including nonwoven fabrics and paper.
For the purposes of describing this invention, and as used herein, the terms thread, string, strand and tape will be used interchangeably in describing the "secondary" material that is bonded to the base substrate. When used, such terms are intended to apply in their broadest sense, as any thread or tape-like construction comprising a plurality of filaments or fibers, or a plurality of thread-like units made up of multiple ends or filaments. This invention contemplates the use of any "secondary" material in strand or thread form, as defined, which exhibits a relatively high degree of tensile strength when compared to the underlying substrate, and is capable of being pressed or flattened onto a base substrate material.
The basic thread/strand/string/tape of the secondary material (commonly referred to as the "core") is uniformly coated and/or impregnated with a thermoplastic adhesive (commonly referred to as "hot melt"). The specific core and hot melt materials which comprise the secondary material for any particular application are respectively selected for their intended end use requirements. The thermoplastic adhesives are solids at normal (ambient) temperatures, but become soft as they are heated and will flow at elevated temperatures. The core/hot melt material is commonly bonded to the base substrate material by a thermocompression bonding technique. This technique comprises the fundamental steps of:
(1) heating the thermoplastic adhesive bearing core material until the adhesive exceeds its "flow point," so that it can "wet" onto the substrate; PA1 (2) applying the heated core/hot melt material to the substrate; PA1 (3) applying pressure to the applied core/hot melt combination on the substrate, thus flattening out the core material and spreading the hot melt relative thereto on the substrate, and PA1 (4) allowing the hot melt to cool, thereby bonding the core material to the substrate.
This process is typically used for applying continuous such secondary strengthening materials to continuous substrates which are being processed through machinery at speeds typically ranging between 25 to 1,000 feet per minute.
A number of differing ways of performing the various individual steps of the thermocompression bonding cycle have been practiced in the past. For example, the hot melt has been heated to its flow point through conduction by passing the core/hot melt material through electrically heated tubes or capillaries, or over an electrically heated wheel or roller; through convection, by subjecting the core/hot melt material to currents of pressurized heated air; and through a combination of the conduction and convection techniques. Similarly, various techniques for pressing the heated core/hot melt material onto the moving substrate have been practiced, such as pressing the core/hot melt and substrate together between a pair of rollers forming a nip or by pressing the core/hot melt onto a substrate with cold or heated plate members. Such prior art techniques, however, have typically used complex or special-purpose structure and have either been too costly, too slow or too inflexible to accommodate changing use applications, generally did not form reliable bonds and generally did not give the degree of control required over the bonding variables to effect quality bonds in a number of different applications.
For example, a common problem associated with such prior art bonding apparatus is that a significant time interval exists between that instant of time at which the heated core/hot melt material leaves the heating apparatus and that instant of time at which the core/hot melt material is pressed onto the substrate. During that time interval, the hot melt cools, thus reducing its wetting, and thus bonding capability. To insure that the hot melt remained above its flow point until it was pressed onto the substrate, such prior art apparatus required the core/hot melt to be heated to temperatures significantly above the flow point of the hot melt, to compensate for the cooling delay. This over-compensation would often result in excessive flow of the hot melt in the heating apparatus, leading to clogging of the heater duct and possible charring of the hot melt and/or melting of the core material if the bonding apparatus were to temporarily stop (i.e. if the core material were to stop advancing through the bonding apparatus). Further, to attain the over-compensated temperature when bonding to fast-moving substrates, the prior art structures required undue complications in the design of their heating chambers. Another deficiency of prior art bonding apparatus is the complex and special purpose nature of their pressure applying/bonding apparatus. For example, those structures having large special-purpose bonding heads for applying core/hot melt compositions to veneers on a horizontal plane, cannot generally be easily modified or adapted to apply such core/hot melt materials to tapes or to other substrates advancing in a vertical plane, or over a differently shaped surface underlying the substrate. A further problem associated with those prior art structures applying a nip to press the core/hot melt material onto the substrate, is that due to the tangential "point-contact" nature of the nip formed between two rollers, such structures have not insured adequate dwell time of the bonding force upon the core/hot melt, for insuring quality bonds. This problem becomes more acute as the substrate speed increases.
The present invention overcomes many of the above-mentioned shortcomings of prior art methods for bonding continuous core/hot melt materials to moving substrates. The present invention provides a simple, efficient and highly flexible apparatus for providing high quality bonds of core/hot melt materials to a wide variety of moving substrates, and is readily adaptable to changing bonding conditions and substrate speeds.